From a Circling Complex Number to
the Simple Harmonic Oscillator

Michael Fowler

Describing Real Circling Motion in a Complex Way

We’ve seen that any complex number can be written in the
form z=reiθ,
where r is the distance from the origin, and θ is the angle between a line from the origin to
z and the x -axis. This means that if we have a set of
numbers all with the same r, but different θ’s,
such as reiα,reiβ,
etc., these are just different points on the circle with radius r centered at the origin in the complex plane.

Now think about a complex number that moves as time goes on:
z(t)=Aeiωt.

At time t,z(t) is at a point on the circle of radius A at angle ωt to the x -axis.
That is, z(t) is going around the circle at a steady angular
velocity ω. We can also write this:

z(t)=Aeiωt=Acosωt+iAsinωt

and see that the point z=x+iy is at coordinates (x,y)=(Acosωt,Asinωt).

The angular velocity is ω,
the actual velocity in the complex plane is dz(t)/dt.

Exercise: what are the x and y components of this velocity regarded as a
vector? Show that it is perpendicular to
the position vector. Why is that?

This differential equation has real and imaginary parts on
both sides, so the real part on one side must be equal to the real part on the
other side, and the same for imaginary parts. That gives

ddtcosωt=−ωsinωt,ddtsinωt=ωcosωt

so differentiating the exponential is consistent with the
standard results for trig functions.

Differentiating one more time,

d2dt2Aeiωt=−ω2Aeiωt.

Again going to the picture of a complex numbers as a two-dimensional
vector, this is just the acceleration of an object going round in a circle of
radius A at angular velocity ω, and is just Aω2 towards the center of the circle, the familiar
rω2=v2/r. Thinking physics here, this is the motion of
an object subject to a steady central force.

Follow the Shadow: Simple Harmonic Motion

But what if we just equate the real parts of both
sides? That must be a perfectly good
equation: it is

d2dt2Acosωt=−ω2Acosωt

This is just the x -component of the circling motion, that is, it is the “shadow” of the circling point on
the x -axis.

Forgetting for the moment about the circling point, and
staring at just this x -axis equation, we see it describes the motion
of a point always having acceleration towards
the origin (that is, the minus sign ensures the acceleration is in the opposite direction to that of the point
itself from the origin) and the magnitude of the acceleration is proportional
to the distance of the point from the origin.

In fact, motion of this kind is very common in nature! It is called simple harmonic motion.

Mass Hanging on Spring

A simple standard example is a mass hanging on a
spring. If it is initially at rest, and
the spring has length L (stretched from its natural length to balance mg ) then if it is displaced a distance x from that equilibrium position, from Hooke's
Law the spring will exert an extra force −kx and the equation of motion will be

md2xdt2=−kx.

This is exactly
the equation of motion satisfied by the “shadow” on the x -axis of the point circling at a steady rate.

The general solution is x(t)=Acos(ωt+δ),
where a possible phase δ is included so that the point can be anywhere
in its oscillation at t=0.